iommu: Support MMU500 Deep Prefetch errata workaround

If deep prefetch is enabled, but not all page table entries in the
prefetch window are valid, improper permissions may be cached for
valid entries in this window. Ensure the start and end of all mapped
buffers are aligned to 16kB to prevent this issue.

Change-Id: I53bc18529b77c775338c477487333b404b225c58
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>